3D Morphology of Fracture Surfaces Created by Mixed-mode II+III Fatigue Loading in Metallic Materials

This paper presents results of 3Dfractographical analysis of shear-mode fatigue cracks in ARMCO iron, α-titanium, nickel and the X5CrNi18-10 steel. Observation of crack paths on a microscopic level enables us to reveal local loading modes at the crack tip under various kinds of remote loading. Experiments were done on special specimens loaded in mixed mode II+III in the near- threshold regime. Only small deviations from the plane of maximum shear stress were observed in ARMCO iron. In titanium, nickel and stainless steel the mean deviation angles were higher. In the stainless steel, the mixed-mode II+III cracks propagated in local mode I, forming a mixture of morphologies of a highly deflected mode II facets and a factory roof in mode III. Crystallographic facets were observed in all materials except for the stainless steel. The results provide a basis for an assessment of micromechanisms of mixed-mode II+III crack propagation and the related near-threshold intrinsic resistance to crack growth.